Circuit for amplifying electrical oscillations with a constant amplification factor



Dec. 30, 1952 J. J. z. VAN ZELST 2,623,955

CIRCUIT FOR AMPLIFYING ELECTRICAL OSCILLATIONS WITH A CONSTANTAMPLIFICATION FACTOR Filed Aug 2, 1948 JOHANNES JACOBUS ZAA LBERG VAI?!ZELST INVENTOR AGENT Patented Dec. 30, 1952 CIRCUIT FOR AMPLIFYING'ELECTRICAL OSCILLATIONS WITH. A CONSTANT AM- PLIFICATION FACTOR JohannesJacobus Zaalber Netherlands, assignm- Bank and Trust Company, Hartford,Conn., as

trustee Application August 2, In the Netherlan 5 Claims.

This invention relates to an improvement in or modification of theinvention described and claimed in my copending application SerialNumber 684,071, filed July 16, 1946, now issued as Patent No. 2,544,132,May 22, 1951. In said copending application a circuit-arrangement isdescribed for amplifying electrical oscillations with the use ofelectric discharge tubes, the ratio between the output and the inputvoltages of the amplifier (amplification factor) having a constant orsubstantially constant value.

The above-identified copending application describes, inter alia acircuit-arrangement which self oscillates at an auxiliary frequencylying without the frequency range of the oscillations to be amplified,said auxiliary frequency having derived from it, by detection a controlvoltage, with the use of which the amplification factor of the amplifieris controlled.

In this case the amplification factor of the circuit arrangement isfound to vary only with the value of the impedances included in thefeedback circuit for the auxiliary frequency and of the impedancesincluded in the amplification channel of the frequency to be amplified.

The invention relates to a circuit-arrangement in which the influence ofvariationsor divergence of theseimpedances at given amplification is cutdown to a minimum value.

According to the invention the feedback circuit for the auxiliaryfrequency is constituted by a phase-shifting network which is built upfrom resistances and reactances and which in addition to the anoderesistance of the discharge tube comprises at least one furtherresistance these resistances and reactances being proportioned to bevsuch that the quotient of the transfer 1mpedance of the network in the.proximity of the auxiliary frequency and the output resistance may berepresented approximately by a whole 5- negative power of a linear formin the frequency. A more detailed discussion of the theory underlyingthe present invention, as well as a full mathematical treatment thereofmay be found in the article appearing in the Philips Technical Review,vol. 9, No. 9, pages 309 to 315, published March 1948 and entitledConstant amplification in spite of changeability of the circuitelements.

In order that the invention may 1y understood and readily carried intoeffect, it will now be described more fully with reference to theaccompanying drawing, in which Fig. 1 shows an oscillator circuit knownper se which comprises a discharge tube l, represented, for

be more clear- 5 g van Zelst, Eindhoven, to Hartford National 1948,Serial No.. 41,926 (is August 25, 1947 the sake of simplicity by atriode, the anode circuit of which is connected through a phase-shiftingnetwork to the grid circuit. This phaseshifting network has several, say3, sections comprising the output resistance H and a capacity 2, aresistance 3 and a capacity 4, a resistance 5 and a capacity 6. Thiscircuit will self-oscillate at a frequency such that the totalphase-shift effected by this network for this frequency is equal to 180.In order to reduce conductance variations of the tube I, theoscillations produced are detected with the use of,

for example, a diode I and its output filter 8 and the voltage producedacross this output filter 8 is then fed, through a grid leak 8, to thegrid of the discharge tube l. A blocking condenser for the direct anodevoltage is designated Ill.

Owing to this measure the mutual conductance of the discharge tube Ivaries only with the value of the impedances included in thephase-shift- 'ing network, since for self oscillation the con- ,and theoutput resistance r in dition applies that the mutual conductance S ofthe tube is adjusted to be such that SZ=1, in which Z designates thetransfer impedance (i. e. the ratio between the ouput voltage and theinput current) of the feedback network. A variation of the impedancesincluded in the feedback circuit or a divergence between the relativevalues of these impedances with various identical amplifiers will beresponsible for the fact that in spite of the control of the mutualconductancev as a function. of the amplitude of the oscillationsproduced, the amplification factor Sr of the circuit arrangement (inwhich r desighates the value of the output resistance II) is notadjusted to the same value.

' According to the invention, the phase-shifting network. is built up insuch manner that the quotient Z/r of the transfer impedances Z theproximity of the auxiliary frequency produced may approximately bedenoted by Z/r=(c1+7'c2w) in which or and=c2 are constants, n. aconstant whole number and w denotesfrequency.

The transfer impedance Z of an electric quadripole may generally berepresented by the quotient. of two higher-power functions of. thefrequency the coefficients of which are functions of the impedancesincluded in the quadripole. If in adidtion to the output resistance r ofthe discharge tube such a quadripole comprises at least one: furtherresistance, as explained in the above-identified publication calculationreveals that, if in the proximity of the auxiliary frequency Z/r may berepresented approximate- 1y by (ci+;ic2w)" a small variation of one ofthe impedances of the phase-shifting network results in that for thisauxiliary frequency (at which the network consequently has to perform aphase-shift equal to a whole multiple of 180) the amplification factorSr of the circuit-arrangement has remained substantially unaltered.

The phase-shifting network may bebu'ilt up more particularly fromsections as shown in the figures. In this case, the above feature, tothe neglect of the relative load, is apparent from that in the proximityof the auxiliary frequency each section produces an equal phase shift ofthis frequency. It is consequently found that in this case the quotientZ/r of the phase-shifting network varies least with small variations ofdivergence of the value of these impedances.

Fig. 2 shows, by way of example, one form of circuit-arrangementaccording to the invention, the capacity 6 of the phase-shifting networkbeing connected in series with an oscillatory circuit I2, to which theoscillations to be amplified are fed by way of input terminals I 3, and.the capacity 2 of the phase-shifting network being connected in serieswith a circuit I4, from which the output oscillations are taken. Sincethe circuits I2 and I4, which are tuned to the frequency of theoscillations to be amplified, exhibit substantially no impedance for theauxiliary frequency, this impedance will scarcely influence theauxiliary frequency. I

In a circuit-arrangement comprising a discharge tube and aphase-shifting network built up from three sections, as shown in Fig. 2,the phase-shift per section is, if the sections load one another to asmall extent, equal to.60 and Z/r equal to A The amplification realizedwith such a circuit-arrangement is thus equal to 8. If the sectionsloadone another any value of the amplification factor between 4 and 1-0 isobtainable in the case of suitable proportioning. Provision must be madeby suitable proportioning of the impedances, for example Of theresistance 9 and the capacity III, to see that the circuit-arrangementdoes not self oscillate at a frequency lower than the desired auxiliaryfrequency.

What I claim is:

1. A circuit arrangement for amplifying signals lying within apredetermined frequency range with constant gain comprising anoscillating and amplifying system for said signals provided with meansto control the amplification thereof and including an electron dischargetube having a cathode, a grid and an anode and an output resistanceconnected to said anode, feedback means for sustaining said system inoscillation at an auxiliary frequency lying outside said predeterminedrange and including an aperiodic phase-shifting network coupled betweensaid anode and said grid and formed by resistance-reactance sectionsconnected in cascade relation, said output resistance constituting theresistance in one of said sections, means to detect the auxiliaryfrequency oscillations generated by said system to produce a controlvoltage, and means to apply said voltage to the amplification controlmeans of said system to maintain constant gain.

2. A circuit arrangement for amplifying signals lying within apredetermined frequency range with constant gain comprising anoscillating and amplifying system for said signals provided with meansto control the amplification thereof and including an electron dischargetube having a cathode, a grid and an anode and an output resistanceconnected to said anode, feedback means for sustaining said system inoscillation at an auxiliary frequency lyin outside said predeterminedrange and including a phase-shifting network coupled between said anodeand said grid and formed by resistance-reactance sections connected incascade relation, said output resistance constituting the resistance inone of said sections, said network being characterized in the vicinityof said auxiliary frequency by a value of transfer impedance which whendivided by the value of said output resistance may be representedapproximately by (oi-Honor where m and G2 are constants, n is a constantwhole number and w denotes frequency, means to detect the auxiliaryfrequency oscillations generated by said system to produce a controlvoltage, and means to apply said voltage to the amplification controlmeans of said system to maintain constant gain.

.3. An arrangement, as set forth in claim 2, in which the reactances insaid sections of said phase-shifting network are constituted bycapacitances and wherein said sections have resistancecapacitance valuesproducing a phase-shift for the auxiliary frequency which issubstantially equal for all sections.

4. An arrangement, as set forth in claim 2, wherein said network isconstituted by three resistance-capacitance sections each producing a 60degree phase-shift.

5. A circuit arrangement for amplifying signals lying within apredetermined frequency range with constant gain comprising anoscillating and amplifying system for said signals including an electrondischarge tube having a cathode, a grid and an anode, a resonant inputcircuit connected to said grid for applying the signals thereto and anoutput resistance connected to said anode, feedback means for sustainingsaid system in oscillation at an auxiliary frequency lying outside saidpredetermined range and including a phaseshifting network coupledbetweensaid anode and said grid and formed by resistance-capacitancesections connected in cascade relation, one of said sections beingconstituted by said output resistance, said network being characterizedby a value of transfer impedance which when divided by the value of saidoutput resistance may be represented approximatelyby (c1+;iczw)", where01 and 02 are constants, n is a constant whole number and w denotesfrequency, a detector-coupled to said output resistance torectify theauxiliary frequency oscillations developed thereacross, means to filterthe rectified oscillations to produce a control voltage, and means toapply said control voltage to said grid to maintain constant gain.

J OHANNES JACOBUS ZAALBERG VAN ZELST.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,237,409 Burnside Apr. 8, 19412,279,128 Paslay Apr. 7, 1942 2,296,626 Blumlein Sept. 22', 19422,346,545 Anderson Apr. 11, 1944

